NLOphoric unsymmetrically substituted D-π-A benzodifuranone dyes: Density functional theory, time dependent-density functional theory, and non-linear optical studies

The linear and non-linear optical properties of thirty-five unsymmetrically substituted compounds are studied using density functional theory and time-dependent density functional theory methods (B3LYP/6311++G(d,p), CAM-B3LYP/6311++G(d,p) and M06-2X/6311++G(d,p)). A comparable set of functionals is also used to investigate vertical excitation. It is observed that the inclusion of the nitrogen-substituted electron-donating group along with the electron-withdrawing group led to more red-shifted absorption maxima and exhibited an excellent (non-linear optical) NLO response. The geometrical framework, dipole moment, and other descriptors, HOMO-LUMO energy gaps, linear polarizability, first-order and second-order hyperpolarizability are calculated to investigate the effect of different electron donating and accepting substituents on the NLO properties of benzodifuranone chromophores. In cases where nitrogen substituents are added, the electron density is more widely spread throughout the donor in HOMO and more displaced to the acceptor in LUMO. The computed first-order and second-order hyperpolarizability values and decreasing HOMO-LUMO energy gaps in disubstituted nitrogen-containing compounds show that 1a-1e, 4a-4e, 5a-5e are promising candidates in all functionals for better NLO properties.